The present invention relates generally to vehicle wheel alignment sensors of the type which are pendulously secured to a vehicle wheel by a mounting shaft during a vehicle wheel alignment procedure, and in particular, to an apparatus and method for identifying and maintaining tracking of an absolute mounting shaft rotational position of the vehicle wheel alignment sensor after mounting to a vehicle wheel.
Computer controlled vehicle wheel alignment systems, such as those shown in U.S. Reissue Pat. No. 33,144 to Hunter et al., U.S. Pat. No. 4,381,548 to Grossman et al., and U.S. Pat. No. 5,598,357 to Colarelli et al., utilize a number of wheel-mounted alignment sensors, such as those described in U.S. Pat. No. 4,879,670 to Colarelli, to obtain measurements related to vehicle wheel alignment parameters. The majority of vehicle wheel alignment sensors currently utilized in the market are “cordless”, relying on internal rechargeable batteries to power associated circuitry, and communicating to a console wheel alignment computer using conventional wireless communications technology. One example of a conventional computer controlled vehicle wheel alignment system is the Series 811 console, which utilizes the DSP-500 series cordless vehicle wheel alignment sensors, and is sold by Hunter Engineering Company of Bridgeton, Mo.
It is known in the industry that vehicle wheel alignment sensors which are pendulously secured to individual vehicle wheels must be compensated for any runout present between a plane in which the vehicle wheel alignment sensor hangs, and a plane perpendicular to the rotational axis of the wheel. The preferred procedures for obtaining runout compensation generally involve mounting a vehicle wheel alignment sensor to a vehicle wheel using a wheel clamp, rotating the wheel and mounting shaft to three distinct rotational positions relative to the sensor housing, and obtaining sensor readings for each position. Using the three sensor readings, a sinusoidal pattern representative of the amount of runout present between the vehicle wheel alignment sensor and the vehicle wheel may be calculated for any rotational position of the vehicle wheel and/or sensor. This runout compensation procedure for a vehicle wheel alignment sensor is described in detail in U.S. Pat. No. 5,052,111 to Carter et al.
Once the runout compensation procedure has been successfully completed, the vehicle wheel alignment sensor establishes a relative base rotational position of the mounting shaft. Utilizing an inexpensive relative rotational position sensor, the vehicle wheel alignment sensor tracks the rotation of the mounting shaft relative to the base rotational position. By tracking the change in the rotational position of the vehicle wheel alignment sensor from the base position, a runout compensation value for the current rotational position of the vehicle wheel alignment sensor is calculated from the previously obtained sinusoidal pattern.
One drawback to using inexpensive relative rotational position sensors is an inability of the sensor to identify an absolute rotational position of the vehicle wheel alignment sensor if the established base rotational position is lost. The established base rotational position in a conventional vehicle wheel alignment sensor can become lost for a number of reasons. For example, if the rechargeable batteries supplying power to maintain the wheel alignment sensor memory fail, or require replacement or recharging, data stored in the memory such as the established base rotational position and sinusoidal pattern will be lost, requiring an operator to repeat the time consuming compensation procedure before vehicle wheel alignment can be resumed. Similarly, in rare cases, battery supplied power can be lost momentarily due to poor or unclean battery contacts.
Even if the data values are stored in a persistent memory, such as one receiving power from a capacitor, which will maintain the data values for a limited period of time until the restoration of the normal power supply, any relative rotational movement between the vehicle wheel alignment sensor, mounting shaft, or vehicle wheel will not be recorded by the relative rotational position sensor, resulting in a discrepancy between the rotational position in which the sensor was compensated, and the current rotational position as identified by the relative rotational position sensor upon restoration of power. Finally, if an operator desires to suspend work on a vehicle in the middle of a vehicle wheel alignment procedure, and shuts down the alignment system (such as overnight), the stored data may be lost, and any rotational movement of the mounting shaft relative to the vehicle wheel alignment sensor will not be tracked, requiring the runout compensation procedures to be repeated upon the subsequent system startup.
It is known that an absolute rotational position sensor which relies upon unique identification markings associated with the mounting shaft to identify the current absolute rotational position of a fixed point on the mounting shaft relative to the vehicle wheel alignment sensor may be utilized in place of the relative rotational position sensor in a cordless vehicle wheel alignment sensor. However, to align modern vehicles, a very high degree of precision is required in the sensor rotational position measurements. When utilizing an absolute rotational position sensor in such a high precision environment, the conventional absolute rotational position sensor must be capable of identifying rotational positions to the same degree of accuracy, and therefore requires a number of unique markings proportional to the required degree of accuracy. Absolute rotational position sensors with unique markings on the mounting shaft which are capable of measuring rotational positions to the required accuracy levels for vehicle wheel alignment are delicate and costly items, and are generally unsuited for use in a vehicle service environment.
Accordingly, there is a need in the industry for an alternative device and method for maintaining cordless vehicle wheel alignment sensor runout compensation values and rotational positions following momentary or extended losses of power, which do not rely upon the use of delicate and costly absolute rotational position sensors with associated markings on the mounting shaft of the vehicle wheel alignment sensor.